Electronic toilet tank monitor utilizing a bistable latching solenoid control circuit

ABSTRACT

A fluid control circuit system is capable of maintaining fluid within a fluid tank at a desired level using electronic sensors and control circuitry, where the control circuitry and actuators are configured for low power consumption, thus allowing operation to be powered by a self contained internal power supply. To provide appropriate fluid control, the system includes a fluid sensor indicating if fluid is at a predetermined level, control circuitry attached to the fluid sensor, a latching solenoid attached to the control circuitry and also attached to a fluid control valve, and an internal power supply to power all electrical components.

This application claims the benefit of Provisional Application No.60/892,359 filed Mar. 1, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic toilet tank monitorutilizing a bistable latching solenoid control circuit to operatesolenoid actuated valves. More generally, the present invention relatesto a control circuit for controlling bistable latching solenoids used tocontrol actuated valves.

2. Discussion of the Related Art

Certain common flush toilets include a water tank positioned above atoilet bowl. The tank holds enough water so that when the water in thetank is released into the bowl fast enough, the water will activate asiphon in the drain line of the toilet. In addition to requiring acertain volume of water, it is critical that the water is released intothe bowl within a relatively small time frame, generally about 3 secondsin order to activate the siphon to flush the water out of the toiletbowl and into the drain pipe. After flushing the water out of thetoilet, it is necessary to again fill the tank with the same volume ofwater. Current tank level controls on toilets use mechanical means toachieve the desired amount of water in the tank.

The flush mechanisms include a handle on the exterior of the tank thatis mechanically coupled to a chain, which is connected to a flush valvewithin the tank. When a user pushes on the handle, the chain is pulled,thereby lifting the flush valve. This moves the flush valve out of theway, revealing a drain hole that is generally about 2- to 3-inches(5.08- to 7.62-cm) in diameter. Uncovering the drain hole allows thewater to enter the toilet bowl. In addition to the volume of water inthe tank and the diameter of the drain hole, the height of the water inthe tank impacts the speed with which the water is released from thetank into the toilet bowl.

In many toilets, the toilet bowl has been molded so that the waterenters the rim, and some of it drains out through holes in the rim. Agood portion of the water flows down to a larger hole at the bottom ofthe bowl. This hole is known as the siphon jet. It releases most of thewater directly into the siphon tube. Because all of the water in thetank enters the bowl in about three seconds, it is enough to fill andactivate the siphon effect, and all of the water and waste in the bowlis sucked out.

Once the tank has emptied, the flush valve is repositioned over thedrain hole in the bottom of the tank, so the tank can be refilled withwater. A refill mechanism is then used to refill the tank to apredetermined height so it is ready for the next flush. The refillmechanism includes a valve that turns the water on and off. In currenttoilets, the valve is controlled by a filler or ball float. When thewater level in the tank is low, the filler float or ball float falls.The valve is thereby opened in order to refill the tank and the toiletbowl. As the water level in the tank rises, the filler float or ballfloat also rises. Once the water level has reached the desired height asdetermined by the buoyancy of the float, the valve is switched into theclosed position. An overflow tube within the tank allows excess water inthe tank to be drained into the bowl to prevent the tank fromoverflowing.

In alternative embodiments, level indicators are electromechanicaldevices that work in combination with some control circuits, systems,and the like. Naturally, these types of devices require electrical powerto operate. However, the known mechanical design used for refillmechanisms (discussed above) does not require electrical connections atthe toilet. As such, existing toilets are not equipped with a constantpower source. Further, bathroom facilities do not presently includepower source which would be convenient to the installed toilet (such asoutlets in close proximity). In addition, electro-mechanical levelindicators used in toilet tank refilling mechanisms must function evenduring power outages. Based on the foregoing, there is a need for atoilet tank water control system that does not require a constantexternal power supply.

Based on the high frequency of toilet use, there exists a need for amechanically reliable toilet tank water control system that can beoperated at low power consumption levels.

Solenoids are well known electromechanical devices used to convertelectrical energy into mechanical energy and particularly into shortstroke mechanical motion. As such, solenoids have long been employed toactuate valves in response to an electrical signal. Typical applicationsof these solenoid valves include controlling fluid flow, gas flow, andthe like. Conventional (non-latching) solenoids require a continuousenergized state to maintain actuation.

To decrease the power dissipated by the solenoid, and particularly inapplications where the solenoid is to be retained in the actuatedposition for significant time periods, latching mechanisms can be usedto hold the mechanical output of the solenoid in one position or theother without requiring continuous power to the solenoid. Self-latchingsolenoid actuated valves are known in the art. Despite advances inself-latching solenoid actuated valves, there continues to be a need forsmaller, faster acting self-latching solenoid actuated valves with lowpower consumption.

Bistable actuators have been used to provide some reduction in powerconsumption. With the introduction of new actuator designs, there hasbeen the introduction of new control circuitry. Some known circuits forcontrolling bistable actuators have been integrated into actuatorsintended to replace conventional solenoid actuators for controllingwater flow. While these integrated latching actuators consumesubstantially less power in the actuated state than conventionalsolenoid actuators, input signals to the latching actuators must remainon at all times in order to keep the actuators in position. Maintainingthe coil of the actuator in an energized state in order to maintain theactuator in a predetermined position increases overall powerconsumption. Accordingly, there exists a need for a bistable latchingsolenoid control circuit with minimal power requirements for actuating awater flow valve.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a refill mechanismthat can reliably control water level in toilet tanks by controlling theinflow of water. Such a refill mechanism will receive with input signalsprovided by toilet tank level indicators appropriately positioned in thetank to signal when predetermined water levels exist. It is anotherobject of the present invention to provide a toilet tank water controlsystem that does not require a constant power supply. It is yet anotherobject of the present invention to provide a mechanically reliabletoilet tank water control system that can be operated at low powerconsumption levels. It is still another object of the present inventionto provide smaller, faster acting self-latching solenoid actuated valveswith low power consumption. It is also an object of the presentinvention to provide a bistable latching solenoid control circuit withminimal power requirements for actuating a water flow valve.

The present invention achieves many of the above-referenced advantagesby utilizing a control system and control components which arespecifically designed for power consumption concerns. More specifically,a bistable latching solenoid is utilized as the control for opening andclosing a related water or fluid valve. By using a bistable latchingsolenoid, the valve can be opened and closed using small pulse signalsfrom the control system. Most significantly, the control system is notrequired to continuously energize the solenoid, thus operating in a moreenergy efficient manner. In addition, the control circuitry is alsospecifically configured to conserve power and operate in an energyefficient manner.

In addition to the power concern outlined above, fluid level sensing isachieved in a relatively straightforward and efficient manner. In oneembodiment, this includes the use of two probes exposed within the tankcapable of differentiating between the existence of fluid versus theexistence of air. As such, when fluid covers both probes, the resistancetherebetween changes which is detectable by the control circuitry.Naturally, other alternative fluid sensors could be utilized.

These and other objects and advantages of the present invention areaccomplished by the toilet tank electronic monitor and bistable latchingsolenoid control circuit in accordance with the present invention. Theinvention will be further described with reference to the followingdetailed description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of an overall fill tubeassembly for the toilet tank electronic monitor in accordance with thepresent invention;

FIG. 2 is top perspective view of the overall fill tube assembly of FIG.1 shown mounted in a toilet tank;

FIG. 3 is a perspective view of a wired printed circuit board assemblyconnected to a power supply in accordance with the present invention;

FIG. 4 is a perspective view of the printed circuit board assembly ofFIG. 3 shown wired to a solenoid;

FIG. 5 is a perspective view of one embodiment of the printed circuitboard in accordance with the present invention;

FIG. 6 is an illustration of one embodiment of the operation of alatching valve in accordance with the present invention;

FIG. 7 is a schematic block diagram of the bistable latching solenoidcontrol circuit;

FIG. 8 is a schematic circuit diagram of the bistable latching solenoidcontrol circuit of FIG. 7; and

FIGS. 9( a)-(c) are timing diagrams of power preconditioning based onthe various input signals.

DETAILED DESCRIPTION

A toilet tank electronic monitor 10 in accordance with the presentinvention senses the presence or absence of water, i.e. the water level,in a toilet tank 28 (FIG. 2) using detection pins. This detectionmethodology is thus used to control at least one flow valve via acontrol circuit. Referring to FIG. 1, the toilet tank electronic monitor10 in accordance with the present invention includes a fill tubeassembly 12, a valve 14, a solenoid 16 and a control box 18. Fill tubeassembly 12 includes a water conduit 20, a water inlet end 22 and avalve inlet end 24. While water conduit 20 is depicted as a generallytubular shape in the figures, those skilled in the art can appreciatethat water conduit 20 can have various shapes and sizes to accommodatewater feed to valve 14. A water source is connected to water inlet end22 such that water is supplied from water inlet end 22 through waterconduit 20 into valve inlet end 24. Water is provided to fill nozzle 26only when valve 14 is in the open position. Fill tube assembly 12 can becomprised of any water resilient materials, including but not limited tocopper, polyvinyl chloride (PVC), and the like. The components of filltube assembly 12 can be individual components that are operablyconnected to one another, one integrated assembly, or a combination ofboth.

Referring now to FIG. 2, the toilet tank electronic monitor 10 of thepresent invention is shown mounted in a toilet tank 28. A handle 30 onthe exterior of tank 28 is connected to a flush valve 32 via aconnecting means 34. Connecting means 34 can be a chain, a polymericsegment, a metal pole, or any such device that can be used to connecthandle 30 to flush valve 32 while resisting corrosion and/or degradationdue to being submerged in water. An overflow tube 36 is positionedwithin tank 28 such that tank fill nozzle 26 does not spray waterdirectly into overflow tube 36. However, a portion of the water fillwill be directed to the overflow tube 36 to provide toilet bowl sidewallrinse during the tank refill.

Referring now to FIG. 3, control box 18 is shown with a cover 38 removedto expose a power supply 40. Power supply 40 shown in FIG. 3 is a ninevolt alkaline battery. Those skilled in the art can appreciate thatvarious power supplies can be used, depending on the necessaryrequirements of the system. The present embodiment utilizes any powersupply that provides at least 5 V DC, including but not limited to aplurality of 1.5 V batteries, a DC wall transformer, and the like.

Referring now to FIG. 4, control box 18 is again shown with cover 38removed to expose a printed circuit board assembly (PCBA) 42 therein.PCBA 42 includes level indicators 44 and contains the necessarycircuitry to carry out the control functions of the present invention.PCBA 42 is also wired to solenoid 16 in order to provide appropriatepower signals based on input readings of water level from levelindicators 44. In one embodiment, level indicators 44 utilizecomplimentary metal oxide silicon technology to sense the difference inresistance between air and water. This difference can then be used toestablish a bistable input control for toggling solenoid 16. Thoseskilled in the art can appreciate that different types of levelindicators, including but not limited to laser level indicators, soniclevel indicators, and the like, can be used in accordance with thepresent invention.

FIG. 5 shows more detail of one embodiment of PCBA 42 in accordance withthe present invention. The design and operation of this embodiment ofPCBA 42 is discussed in greater detail below with regard to FIGS. 7-9.Those skilled in the art can appreciate that PCBA 42 can scaled up ordown for use in various water flow valve and /or water level controlapplications.

Referring again to FIG. 1, in one embodiment, valve 14 is a magneticallylatching solenoid valve. In this embodiment, valve 14 may have aninternal diaphragm that can be hydraulically maintained in the openposition. In another embodiment, valve 14 is a custom valve with similaroperating characteristics.

Referring now to FIGS. 1 and 6, in one embodiment, solenoid 16 is 2/2magnetically latching bistable solenoid having a coil resistance of 18±1Ω and an operating voltage range of 6-12 V DC. Solenoid 16 in thisembodiment can operate with latching valve 14 at a power down to 5 V DCand with a pulse width of 0.020 seconds (to close) and 0.060 seconds (toopen). Operation under these parameters maximizes battery life forbistable latching solenoids. In position 1 46 on FIG. 6, if valve 14 isin the closed position and coil is supplied with voltage pulsed current64 having a pulse width of 60 mS at inputs 60 and 62, valve 14 is placedin the open position where it remains until supplied with additionalpower. Supply of additional power is shown in Position 2 48 of FIG. 6.Here, when valve 14 is opened by supplying current as occurs in position1, valve 14 can only be closed by again supplying pulsed current 66.Valve 14 remains in the closed or off position until additional power issupplied again. Further detail regarding this operation is outlined inrelation to the control circuitry discussed below.

Those skilled in the art can appreciate that timing durations, solenoiddriver devices, battery voltage, input control, and the like will bedependent upon application specific “latching solenoids” having uniqueoperational requirements. Because various application specific “latchingsolenoids” can be used to control a variety of different types and sizesof flow valves, one embodiment of a bistable latching solenoid controlcircuit 50 in accordance with the present invention is discussedhereinafter without specifying particular timing durations, solenoiddriver devices, battery voltage, input control, and the like.

Referring now to FIGS. 7 and 8, there is shown a schematic diagram 52 ofbistable latching solenoid control circuit 50. The circled alphabeticalreferences (A) through (L) are used as operational reference pointsreferring to the application of power to circuit 50 and the powerpreconditioning that initializes operation of circuit 50. These circledalphabetical references also correspond to information in the circuitdiagram of FIG. 8 and the timing diagram of FIGS. 9( a)-(c) as follows:“A” represents an input stage. “B” represents an input pulse delay. “C”represents a power preset. “D” represents a two input Schmitt TriggerNAND gate. “E” represents a positive edge triggered one shot pulse. “F”represents a positive edge triggered one shot pulse inverter. “G”represents a positive triggered one shot pulse delay. “H” represents anegative edge triggered one shot pulse. “I” represents a negative edgetriggered one shot pulse inverter. “J” represents a negative edgetriggered one shot pulse delay. “K” represents a latching solenoid line1 for unlatch control. “L” represents a latching solenoid line 2 forlatch control.

Referring in more detail to FIG. 7, schematic diagram 52 illustrates theexistence of an input stage 70 which will receive a latched or unlatchedsignal at its input. Input stage 70 also receives power from battery 100which has its output limited by a current limiting resister 102. Anoutput from input stage 70 is then passed to an input pulse delay 72which will feed one side of a two input Schmitt Trigger NAND gate 76. Inaddition, a power preset circuit 74 supplies a second input to SchmittTrigger NAND gate 76 (in addition to any necessary power signals). Theoutput from two input Schmitt Trigger NAND gate 76 is then provided to apair of one shot pulse generators: negative edge triggered pulsegenerator 78 and positive edge triggered pulse generator 80. As will berecognized, each of these circuits will generate pulses at appropriatetimes in response to received falling or rising edges of pulses,received at the respective input. Connected to the output of negativeedge triggered pulse generator 78 is an inverter 92 along with a pulsedelay circuit 94. Inverter 92 feeds a high side MOSFET switch 98, whilepulse delay circuit 94 feeds a low side MOSFET switch 96. Similarly,outputs from positive edge triggered one shot pulse generator 80 isprovided to inverter 82 and pulse delay 84. Inverter 82 then feeds highside MOSFET switch 86 while pulse delay circuit 84 will feed a low sideMOSFET switch 88. As discussed in greater detail below, each of thesecomponents cooperate with one another to provide appropriate control oflatching solenoid 90.

Referring now to FIGS. 8 and 9( a)-(c), component references (R1, C1,U1, and the like) are used to identify certain components of circuit 50which are configured to carry out the desired operation. Further, thesereferences are also referring to the application of power to circuit 50and the power preconditioning that initializes operation.

Circuit 50 depicted in FIGS. 7-9( a)-(c) is designed using complimentarymetal oxide silicon (CMOS) technology for water level indication andSchmitt Trigger gating to obtain low frequency operation and low powerconsumption ideal for battery applications. Those skilled in the art canappreciate that various level indication and gating technology can beused when designing circuit 50 for various applications, including butnot limited to control of substances other than water.

Circuit 50 performs one of two stable control operations based upon theinput state “unlatch” or “latch” for latching style solenoids. Circuit50 is powered by a single DC power source. When the DC power is appliedto the circuit it will perform a solenoid “unlatch” operation as part ofits power preconditioning initialization state. After the powerpreconditioning operation the circuit will respond to its input state.If the input state is “unlatch” then no further operation is performed.If the input state is “latch” then the circuit will perform the “latch”solenoid operation routine.

The “unlatch” and “latch” input control commands each initialize onefixed pulse to trigger the bistable latching solenoid. The input pulseis time delayed which limits how fast circuit 50 can toggle between thetwo input control states preventing both circuit paths fromsimultaneously actuating the solenoid operation. Bistable control of thelatching solenoid requires bi-directional electrical current. In betweena change of input states, circuit 50 will default to sleep mode for lowpower consumption.

Referring now to FIGS. 9( a)-(c) there is depicted a timing diagramwhich illustrates operation in accordance with the design of circuit 50in the present invention. T1 through T9 along the top of the FIG. 9( a)are used to identify timing events. The timing events show the specificlogic level states (“0” or “1”) for timing identifiers listed along theleft side of FIGS. 9( a)-(c). These timing identifiers correlate withcircled alphabetical references (A) through (L) and also correspond tolike indicators on FIGS. 7 and 8.

Referring specifically to FIG. 9( a), there is shown a timing diagramfor the application of power to circuit 50 and the power preconditioningthat initializes operation of circuit 50 where the input state is set to“LATCH.” Timing Event T1 represents the application of DC power tocircuit 50. As previously discussed, circuit 50 can be powered by asingle DC power supply source (+V BATT). When power is applied tocircuit 50, input bias voltage level (A) will begin to charge capacitorC2 through resistor R7 (B). Likewise, the applied power will begin tocharge capacitor C3 through resistor R5 (C). In the powerpreconditioning stage, the input to U1C pin 9 will be at logic level “0”(C) until the capacitor C3 charge voltage exceeds the Logic ThresholdValue (LTV) (Timing Event T5). Similarly, until the capacitor C2 chargevoltage exceeds the LTV (Timing Event T6) the input to U1C pin 8 will belogic level “0” (B). As will be appreciated, U1C corresponds to the twoinput Schmitt Trigger NAND gate 76 as illustrated in FIG. 7.

With both inputs to U1C equal to logic level “0” the U1C pin 10 output(D) will be logic level “1” triggering the positive edge triggered “oneshot” pulse (E). (Again, corresponding to pulse generator 80 shown inFIG. 7.) The positive edge triggered “one shot” pulse (E) will begin tocharge capacitor C6 through resistor R12. The inverted positive edgetriggered “one shot” pulse will bias the high side MOSFET Q3 intoconduction (F). The pulse is inverted by UC2 (inverter 82) to providethis signal.

Timing Event T2 represents the beginning of the “UNLATCH” solenoidpulse. This is provided by an appropriate delay using pulse delay 84.Specifically, when capacitor C6 charge voltage exceeds the LTV of theU3A pin 1&2 input the delayed positive edge triggered “one shot” pulse(G) will bias the low side MOSFET Q4 into conduction initializing thelatching solenoid “UNLATCHED” state (K).

Timing Event T3 represents the end of the “UNLATCH” solenoid pulse. Whenthe positive edge triggered “one shot” pulse (E) completes the one pulsetime period it will switch to logic level “0”. The inverted positiveedge triggered “one shot” pulse (F) will bias the high side MOSFET Q3into non-conduction de-energizing the solenoid (K) and causing a “freewheeling current,” or inductive kickback, from the inductive load of thesolenoid.

Timing Event T4 represents dampening of the free wheeling current, orinductive kickback, from the solenoid. The positive triggered “one shot”pulse (E) logic “0” will begin to discharge capacitor C6 throughresistor R12. When capacitor C6 discharge voltage drops below the LTVthe delayed positive edge triggered “one shot” pulse (G) will bias thelow side MOSFET Q4 into non-conduction and the unlatch cycle of thesolenoid is complete. During the time period between T3 and T4 theMOSFET Q4 remains conductive allowing its internal “drain to source”protection zener diode to forward conduct the “free wheeling current”caused by the inductive load of the solenoid.

Timing Event T5 represents the end of power preconditioning. When thecapacitor (C3) charge voltage exceeds the LTV (from Timing Event T1) theinput to U1C pin 9 will be logic level “1” (C). As illustrated,Capacitor C3 and resister R5 correlate to power preset circuit 74. Thecircuit will remain in this state until further events are encountered.

Timing Event T6 represents operation of the solenoid with “LATCH” as theinput command. This change will be in response to a change at the input,thus indicating that fluid is no longer present at the desired level.When the capacitor (C2) charge voltage exceeds the LTV (from TimingEvent T1) the input to U1C pin 8 will be logic level “1” (B). With bothinputs to U1C set to logic level “1” the U1C pin 10 output (D) will belogic level “0” and will trigger the negative edge triggered “one shot”pulse (H), which is generated by the components making up pulsegenerator 78. The negative edge triggered “one shot” pulse (H) willbegin to charge capacitor C4 through resistor R6 of pulse delay 94. Theinverted negative edge triggered “one shot” pulse (inverted by inverter92) will bias the high side MOSFET Q1 into conduction (I).

Timing Event T7 represents the beginning of the “LATCH” solenoid pulse.When capacitor C4 charge voltage exceeds the LTV of the U1D pin 12&13input delayed negative edge triggered “one shot” pulse (J) will bias thelow side MOSFET Q2 into conduction initializing the latching solenoid“LATCHED” state (L).

Timing Event T8 represents the end of the “LATCH” solenoid pulse. Whenthe negative edge triggered “one shot” pulse (H) completes the one pulsetime period it will switch to logic “0”. The inverted negative edgetriggered “one shot” pulse (I) will bias the high side MOSFET Q1 intonon-conduction de-energizing the solenoid (L) and causing a “freewheeling current” (inductive kickback) from the inductive load of thesolenoid.

Timing Event T9 represents dampening of the “free wheeling” current fromthe Solenoid. The negative edge triggered “one shot” pulse (H) logiclevel “0” will begin to discharge capacitor C4 through resistor R6. Whencapacitor C4 discharge voltage drops below the LTV the delayed negativeedge triggered “one shot” pulse (J) will bias the low side MOSFET Q2into non-conduction and the latch cycle of the solenoid is complete.During the time period between T8 and T9 the MOSFET Q2 remainsconductive allowing its internal “drain to source” protection zenerdiode to forward conduct the “free wheeling current” caused by theinductive load of the solenoid.

Referring now to FIG. 9( b), there is shown a timing diagram foroperation of circuit 50 when the input changes to the “UNLATCH” state.Referring now to FIG. 9( c), there is shown a timing diagram foroperation of circuit 50 when the input changes again to the “LATCH”state.

While the invention has been described with reference to the specificembodiments thereof, those skilled in the art will be able to makevarious modifications to the described embodiments of the inventionwithout departing from the true spirit and scope of the invention. Theterms and descriptions used herein are set forth by way of illustrationonly and are not meant as limitations. Those skilled in the art willrecognize that these and other variations are possible within the spiritand scope of the invention as defined in the following claims and theirequivalents.

1. A fluid level monitoring and control system for controlling the level of a fluid within a fluid tank, comprising: a fluid level sensor positioned at a predetermined position within the fluid tank, the fluid level sensor having an output capable of producing an output signal indicative of the presence or absence of fluid; a control circuit connected to the fluid level sensor output, the control circuit responsive the fluid sensor output signal to produce a first preset control signal when fluid is not detected by the fluid sensor and a second present control signal when fluid is detected by the fluid sensor; an internal power supply for providing power to the control circuit; and a bistable latching solenoid operably coupled to the control circuit to receive the first preset control signal and the second preset control signal, wherein the first preset control signal will cause the solenoid to be actuated to a first position and wherein the second preset control signal will cause the solenoid to be actuated to a second position.
 2. The fluid level monitoring and control system of claim 1 wherein the solenoid is attached to a fluid valve such that actuation of the solenoid to the first position caused the fluid valve to be open, and wherein the actuation of the solenoid to the second position causes the fluid valve to be closed.
 3. The fluid level monitoring and control system of claim 1 wherein the bistable latching solenoid has a first input and a second input, and wherein the first preset control signal is a predetermined pulse provided to the first input to cause the bistable latching solenoid to move to the first position, and wherein the second preset control signal is a predetermine pulse provided to the second input to cause the bistable latching solenoid to move to the second position.
 4. The fluid level monitoring and control system of claim 1 wherein the bistable latching solenoid is a magnetically latching bistable solenoid.
 5. The fluid level monitoring and control system of claim 1 wherein the fluid sensor comprises a pair of probes exposed to the fluid, wherein fluid level is detected by measuring the resistance between the pair of probes.
 6. The fluid level monitoring and control system of claim 1 wherein the fluid sensor is a sonic level indicator capable of detecting the difference between air and fluid in close proximity thereto.
 7. The fluid level monitoring and control system of claim 1 wherein the fluid sensor produces a state change at the fluid sensor output when fluid at the desired level is detected, and wherein that state change is detected by the control circuit to thus produce either the first preset control signal or the second present control signal.
 8. The fluid level monitoring and control system of claim 1 wherein the fluid tank is a fill tank for a toilet.
 9. The fluid level monitoring and control system of claim 1 wherein the fluid tank is a manufacturing process supply tank providing fluid to a manufacturing process.
 10. The fluid level monitoring and control system of claim 1 wherein the internal power supply comprises a battery.
 11. A toilet tank fluid level control system for maintaining fluid at a predetermined level, comprising: a fluid sensor positioned at a predetermined level within the toilet tank to monitor the presence of liquid at the predetermined level; a control circuit coupled to the fluid sensor for receiving a signal from the fluid sensor indicative of the presence or absence of fluid at the predetermined level; a magnetically latching bistable solenoid coupled to the control circuit, the latching solenoid capable of being toggled between a first position and a second position, wherein the movement of the solenoid is responsive to a control signal produced by the control circuit; an internal power supply operably coupled to the control circuit and the solenoid to provide operating power therefor; and a control valve attached to a fluid line which provides fluid to the toilet tank when the valve is open, the control valve attached to the latching solenoid such that the valve is open when the solenoid is in the first position, and the valve is closed when the solenoid is in the second position.
 12. A toilet tank fluid level control system of claim 11 wherein the fluid sensor comprises a first probe and a second probe exposable to fluid within the tank and wherein the control circuit is capable of detecting when both the first probe and the second probe are submerged in fluid. 